The present invention relates to a coupler for use with an ultrasonic transducer probe for transmitting ultrasonic waves to or receiving ultrasonic waves from a target region in a sample or object under examination.
Where a living human body is examined for medical diagnosis to observe a superficial organ such as a thyroid gland or a carotid artery with a sector-shaped beam emitted from a sector-type ultrasonic transducer probe, it is necessary to position the focusing point of a lens attached to the tip end of the probe in the vicinity of the observed region. To meet such a requirement, a coupler is mounted on the ultrasonic transducer probe for keeping the probe spaced a prescribed distance from the surface of the living body being examined. Such a coupler is illustrated in FIGS. 8 and 9 of the accompanying drawings. FIG. 8 is a sectional front elevational view of the copuler, the plane as viewed in FIG. 8 being referred to as a scanning plane. FIG. 9 is a sectional side elevational view of the coupler, the plane as viewed in FIG. 9 being referred to as a plane normal to the scanning plane. A coupler C is mounted on the tip end of a sector-scanning ultrasonic transducer probe 11. The coupler C has a mounting surface 2 which is mounted on the probe 11 and an abutting surface 3 which will be held against an object to be examined. The coupler C is in the shape of a rectangular parallelepiped as a whole and has a height h, a length L in the scanning plane, and a width W in the plane normal to the scanning plane.
When the coupler C is employed, the focusing point P of the lens of the probe 11 is closer to the observed region by the height h than focusing point P is when such coupler C is not employed.
Where the target region to be observed is positioned not deeply below the surface of the object living body, it is necessary to use a coupler of a larger height h in order to bring the focusing point P into conformity with the observed region. This causes the following problems:
The first problem is that as the coupler height h is increased, the length L and the width W of the coupler are also increased, resulting in a large coupler size which cannot be handled with ease.
The second problem will be described with reference to FIG. 10, which shows the relationship bewteen gains of the probe and coupler heights according to a STC (Sensitivity Time Control) curve for a signal received by the probe. In case a human body is diagnosed by an ultrasonic transducer probe, a signal received from an observed region by the probe is generally weaker as the region is located more deeply in the body, i.e., as the depth of the region is greater. Such a signal level variation is compensated for by employing the STC curve to correct the received signal. The interior of the coupler is usually filled with water in which ultrasonic energy is substantially not attenuated. Where couplers C1, C2 having different heights h1, h2, respectively, are employed, no problem is caused by the coupler C1 of the smaller height h1 as its gain G1 is small, but the larger gain G2 of the coupler C2 of the larger height h2 amplifies multiple reflection in a member positioned on the surface of the coupler C which contacts the living body under examination. Such amplified multiple reflection results in an artifact on the reproduced image, presenting an obstacle to the proper reading of the image. Therefore, the coupler for use with an ultrasonic transducer probe should not be of a large height.
Japanese Utility Model Laid-Open Publication No. 57-136304 discloses, as shown in FIG. 11 of the accompanying drawings, a linear-scanning ultrasonic transducer probe 11' having an ultrasonic wave transmission/reception surface 15 and combined with a converter 12 with an acoustic lens 16 held against the ultrasonic wave transmission/reception surface 15. The disclosed arrangement is however capable of only shifting the focusing point for transmitted and received ultrasonic waves, but is not based on any idea of keeping the probe and an examined object spaced a prescribed distance from each other.